<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="vue.js"></script>

    <style>
        .fade-enter,
        .fade-leave-to{opacity:0;}
        .fade-enter-active,
        .fade-leave-active{transition:opacity .5s;}
    </style>
</head>
<script src="vue.js"></script>

<body>
    <div id="app">
        <transition name="fade" mode="out-in">
            <button :key="isOff" @click="isOff=!isOff">{{isOff ? 'Off':'On'}}</button>
        </transition>
    </div>
    <script>
        var vm=new Vue({
            el:"#app",
            data:{
                isOff:false
            }
        })
    </script>
</body>
</html>